MQTT: Operator require dataAttributeName even when there is only one attribute

[chanskw]

With the MQTT operators, we used to be support the blob format for a message.
So, the schema of the stream is this:

schema BlockMsg

So, code like this would work:

() as MQTTSink_7 = MQTTSink(BlockMsg as inPort0Alias)
{
param
topic : "data" ;
serverURI : "tcp://server:16102" ;
}

With the introduction of dataAttributeName, this code will no longer compile because we either require the attribute, or have an attribute that is named "data" on the stream.

This breaks source compatibility. In the case where we only have a single attribute, we should simply assume that the attribute is the data attribute.

[chanskw]

Fixed